Manufacture of carbon filaments

ABSTRACT

A process is disclosed for the production of carbon filaments having a very high modulus of elasticity which have utility as structural materials. The process comprises the carbonization and pyrolysis of a filamentary material capable of carbonization with maintenance of its filamentary structure to temperatures above 900* C in which the filamentary material is a multifilament tow and this is heated up to a temperature of at least about 900* C under conditions of imposed linearity to reduce incidence of kinks in the carbon filaments. The linearity is preferably imposed by imparting a twist to the tow. A preferred raw material is a multifilament tow of a polyacrylonitrile polymer or copolymer containing at least 85% of acrylonitrile. The carbon filaments produced by the process of the invention have a reduced incidence of kinks.

United States Patent Tatchell 1 May 16, 1972 [54] MANUFACTURE OF CARBONLimited, London, England [22] Filed: Jan. 31, 1969 [21] Appl. No.:795,719

[30] Foreign Application Priority Data Feb. 21, 1968 Great Britain..5,354/68 [56] References Cited UNITED STATES PATENTS 3,285,696 11/1966Tsunoda ..23/209.1

3,412,062 11/1968 Johnson et al. 260/37 Primary Examiner-Edward J. MerosArtorney-Plumley, Tyner & Sandt 5 7] ABSTRACT A process is disclosed forthe production of carbon filaments having a very high modulus ofelasticity which have utility as structural materials.

The process comprises the carbonization and pyrolysis of a filamentarymaterial capable of carbonization with maintenance of its filamentarystructure to temperatures above 900 C in which the filamentary materialis a multifilament tow and this is heated up to a temperature of atleast about 900 C under conditions of imposed linearity to reduceincidence of kinks in the carbon filaments. The linearity is preferablyimposed by imparting a twist to the tow. A preferred raw material is amultifilament tow of a polyacrylonitrile polymer or copolymer containingat least 85% of acrylonitrile. The carbon filaments produced by theprocess of the invention have a reduced incidence of kinks.

3 Claims, No Drawings MANUFACTURE OF CARBON FILAMENTS BACKGROUND OFINVENTION This invention relates to the production of carbon filamentshaving a very high modulus of elasticity.

Carbon filaments having a very high modulus of elasticity, for exampleabove 20 X p.s.i. and an ultimate tensile strength above 150,000 p.s.i.are known and are capable of important use as structural materials whenincorporated in matrices such as plastics and metal.

The starting material for the production of such carbon filaments isessentially a filamentary metal capable of carbonization withoutdestruction of its filamentary nature. Organic materials which may beused for this purpose include natural and synthetic cellulose, whichterm include esters and ethers of cellulose, and polymers or copolymerof acrylonitrile. The steps in the production of the filament include ingeneral a controlled pyrolysis step to form the carbonaceous materialand a subsequent step of heating to well above the pyrolysistemperature.

In one process of this type a cellulosic material is subjected to acarefully controlled heating schedule in a protected atmosphere. Thisschedule has the aim of raising the temperature slowly from roomtemperature to about 400 C to decompose the chemical structure of thecellulosic material without destroying its filamentary nature, thensomewhat quicker increase of temperature up to about 900 C to removefurther more stable constituents of the pyrolized material and produce afilament component substantially of carbon, and

' finally a rapid heating up to about 3000 C to complete the pyrolysisand generate a stable carbon filament which may contain graphiticconstituents.

In another process of this type a polymer or co-polymer of acrylonitrilepreferably containing at least 85 mole per cent of acrylonitrile iswound on a frame and heated in the presence of oxygen at l90260 C tomodify the chemical structure of the polymer and form a blackenedfilament which can then be pyrolyzed by subsequent heating in aprotective atmosphere, for example argon, up to about 3,000" C to form astable carbon filament which may contain graphitic constituents. Thissubsequent heating may take place in two stages the first involvingheating to about 900-l500 C to produce a product of high strength thesecond optional heating stage being heating to about 3000 C to produce aproduct of high modulus but somewhat lower strength than that productwhich results from the first heating stage.

One difficulty which occurs in the production of carbon filaments by theprocess outlined above is the production of kink free filaments. Theproduction of such filaments is highly desirable because of theapplication of the material as a structural reinforcing element. This isbecause the full benefit of the high strength and high modulusproperties of the filaments is obtained primarily under direct tensileloading of the filaments and is obtained relatively poorly if thefilaments are oriented so that portions thereof lie across the directionof the forces loading the structural component.

The defects in the linearity of the carbon filaments which most commonlyoccur are in the form of bends or kinks of about one-fourth inch inlength and it is an object of the present invention to provide a processfor the production of carbon filaments having a reduced incidence ofdefects in linearity, in order to realize fully the inherent physicalproperties of such materials.

We have found that this may be achieved by carrying out the heating ofthe carbonaceous material forming the filaments under conditions whichimpose a linearity on the material up to temperature of at least about900 C. Thereafter the dimensional changes are relatively small and thereis only a small chance of bends or kinks developing. This is achievedfor example where the raw material is a multi-filament tow ofpolyacrylonitrile by imparting radial compression of the filaments withrespect to the longitudinal axis of the tow during the heating in thepresence of oxygen to modify the chemical structure so as to bond thefilaments together and to continue the heating of the filaments to atemperature of about 900 C with the filaments bonded together in thismanner. It is found that at the end of this treatment step theindividual filaments are substantially free of bends and kinks and maybe separated one from the other.

Accordingly the invention provides a process for the production ofcarbon filaments by the carbonization and pyrolysis of a filamentarymaterial capable of carbonization with maintenance of its filamentarystructure to temperatures above 900 C in which the filamentary materialis a multifilament tow and this is heated up to a temperature of atleast about 900 C under conditions of imposed linearity to reduceincidence of kinks in the carbon filaments.

The radial compression can be imparted to the multi-filament tow in anysuitable manner and may to some extent be inherent in the raw material.One method by which radial compression may be imposed is by imparting acontrolled twist to the tow which causes the outer filaments to exert aradial compression on the inner filaments. A twist within the range ofone twist per every 6 inches to every 2 feet is preferred and inpractice a twist of about one twist per foot has been found to beadvantageous. Other methods which may be used in combination therewithare carrying the decomposition out under tension and restricting theshrinkage of the material at this stage.

When the organic raw material, that is the multi-filament tow ismodified by heating adherence takes place between the carbonaceousfilaments. The radial compression on the material can then be releasedbecause linearity is still imposed on the carbon filaments by theadherence of the filaments to each other. When treatment of thismaterial by heating to at least 900 C is continued the position of eachfilament is fixed with respect to each other so the incidence of bendsand kinks in the individual filaments is reduced. After the heatingtreatment is complete the filaments may be separated from the filamentbundle and are substantially free of kinks. They may if desired befurther processed by the process described in the application of evendate herewith of Kenneth G.O. Dixon et al entitled Improvements in orRelating to the Manufacture of Filamentary Materials by heating to amuch higher temperature in conditions where oxygen and active materialsare eliminated from the environment of the filament.

A preferred raw material for carrying out the process of the presentinvention is polyacrylonitrile polymer or copolymer in the form of amulti-filament tow. Decomposition of which is preferably effected byheating it at approximately 220 C in the presence of oxygen undertension and with an imparted twist. The invention is particularlyapplicable to the production of carbon filaments having a length inexcess of 18 inches.

In order that the invention may be more fully understood, the followingexample is given, by way of illustration only:

EXAMPLE A polymeric filament being a co-polymer of 93 weight percentacrylonitrile, 6 weight percent methyl methacrylate and 1 weight percentitaconic acid, each filament being 1.5 denier and having a tenacity ofnot less than 5.5 grams per denier, the filaments being contained in atow containing 10,000 filaments approximately was wound on a metal frameand heated in the presence of an oxygen containing atmosphere at 220 Cto modify the chemical structure of the polymeric material. The color ofthe material after heating was black. During this heating stage, radialcompression was obtained by applying a tension of about 2 kilogramswhile winding the material onto the frame, by restricting the shrinkageof the material during this heating stage thereby increasing the tensionsharply as volatile products are driven off, and by giving individualfilaments a helical configuration longitudinally of the tow, for exampleby giving the tow one twist every 12 inches..

The blackened filaments were cut off the frames in lengths of 1 meterand were transferred very carefully, retaining the adhesion betweenindividual filaments, to a carbon boat. This stage is critical becauseany bending of the blackened tow can destroy the adhesion between thefilaments. The carbon boat was filled with a large number of suchlengths arranged very carefully parallel to each other and retained inthe boat under slight compression of about 1 kilogram obtained by meansof a carbon lid.

The boat was transferred to an oven and heated to a temperature of 1,000C in a protective atmosphere of nitrogen. Heating was continued untilevolution of cracked products had ceased.

The material so obtained had a diameter of 7 microns, a modulus of 28 X10 p.s.i. and an ultimate tensile strength of 260,000 p.s.i. The productwas substantially straight and free from kinks and could be handledquite freely for incorporation into a structural matrix.

I claim:

1. A process for the production of carbon filaments which comprises thesteps of heating a multi-filament polyacrylonitrile tow to a temperatureof l90-260 C in the presence of oxygen to modify the filaments, saidheating being carried out on the material while it is under tension andafter a controlled twist within the range of one twist per every 6inches to 2 feet has been imparted thereto so as to impart radialcompression to the filaments with respect to the longitudinal axis ofthe tow and cause the filaments to adhere together and transferring themodified filaments to a heating zone and heating to a temperatureof atleast about 900 C in an atmosphere free of oxygen, care being takenduring said transfer to maintain the adhesion between the filaments.

2. A process as claimed in 1 in which a twist of about one twist perfoot has been imparted to the material.

3. A process for the production of carbon filaments which comprises thesteps of heating a multifilament polyacrylonitrile tow to a temperatureof -260 C in the presence of oxygen to modify the filaments, saidheating being carried on the material while it is under tension andafter a controlled twist within therange of one twist per every 6 inchesto 2 feet has been imparted thereto so as to impart radial compressionto the filaments with respect to the longitudinal axis of the tow andcause the filaments to adhere together and transferring the modifiedfilaments to a heating zone, and heating said modified filaments in anatmosphere free of oxygen to a temperature of at least about 900 C, carebeing taken during said transfer to maintain the adhesion between thefilaments, and heating said resulting filaments to a temperaturesubstantially in excess of 900 C in the presence of a carbon getteringmaterial.

2. A process as claimed in 1 in which a twist of about one twist perfoot has been imparted to the material.
 3. A process for the productionof carbon filaments which comprises the steps of heating a multifilamentpolyacrylonitrile tow to a temperature of 190* - 260* C in the presenceof oxygen to modify the filaments, said heating being carried on thematerial while It is under tension and after a controlled twist withinthe range of one twist per every 6 inches to 2 feet has been impartedthereto so as to impart radial compression to the filaments with respectto the longitudinal axis of the tow and cause the filaments to adheretogether and transferring the modified filaments to a heating zone, andheating said modified filaments in an atmosphere free of oxygen to atemperature of at least about 900* C, care being taken during saidtransfer to maintain the adhesion between the filaments, and heatingsaid resulting filaments to a temperature substantially in excess of900* C in the presence of a carbon gettering material.